Hitherto, as lithographic supports, aluminium plates have been widely used. However, with such supports it is necessary to carry out roughing of the surface, so-called graining, for the purpose of improving adhesion to a light-sensitive layer and giving a water retention property. Graining is an important process for producing lithographic plates because it has a great influence upon the aptitude for a plate making process or printing durability in the case of carrying out printing by mounting on an offset printing press after making a printing plate.
Graining can be carried out by mechanical graining which utilizes sand-blasting, ball-graining, wire-graining or brush graining by a nylon brush and abrasives/water slurry, chemical graining which comprises etching a special aluminium alloy with alkali as described in Japanese Patent Application (OPI) No. 61304/76 (The term "OPI" as used herein refers to a "published unexamined Japanese Patent Application".), electrochemical graining as described in, for example, Japanese Patent Applications (OPI) No. 146234/79 and Japanese Patent Publication No. 28123/73, and combinations of a mechanical graining process and electrochemical graining as described in, for example, U.S. Pat. Nos. 4,476,006 and 4,477,317 have been known.
However, in the case of ball-graining which is one of typical mechanical graining processes, there are many factors requiring skill, such as selection of the material of balls, the kind of abrasives, the control of water-amount, and the like, and the operation can not be carried out continuously. In the case of wire-graining the grain texture is not uniform. On the other hand, brush-graining is a process of improving the above described processes, but there are disadvantages in that the texture is generally simple shallow grains and scratches caused by a revolving brush are left on the surface. In addition, directionality of the grains appears and nonimage parts are easily stained.
In the chemical graining process described in Japanese Patent Application (OPI) No. 61304/76, since an aluminium alloy plate containing 1.6 to 2.5% of manganese should be used, there are disadvantages in that it is hard to get such a material or stain appears on the prints depending upon printing conditions and thus deteriorates the quality.
On the other hand, according to an electrochemical graining process, uniform grains having a large average surface roughness as compared with that in the prior mechanical graining processes such as ball-graining or brush-graining, etc. can be obtained. However, it has a disadvantage of having a very narrow condition. When conditions such as the composition of the electrolyte, the temperature thereof, or the electric condition such as electric current density, etc. are kept constant, products having uniform performances can be easily obtained, but the electrolytic conditions therefor are in a very narrow range and it is very difficult to control each condition so as to be in a suitable range. Moreover, when graining of the surface of an aluminium plate is carried out by only the electrochemical graining process, the cost of electric power consumed is very great and the rate of electric power in the production cost becomes great. Therefore, there is a problem from the viewpoint of economy.
On the contrary, in a process wherein brush graining and electrochemical graining are combined, as described in U.S. Pat. Nos. 4,476,006 and 4,477,317, the directionality of grains disappears and uniform roughness is obtained and consumption of electric power is small. However, if the brush graining is carried out for a long time with one brush, there are problems on continuous operation, namely, uniform quality can not be obtained because of abrasion by the brush, and operation should be stopped when the brush is worn out. Moreover, there is a problem that only a lithographic printing plate easily causing stain on the nonimage part is obtained because of the influence of brush-graining.